Fluid reaction propulsive device



y 1949- T. D. GREGG 2,475,022

FLUID REACTION PROPULSIVE DEVICE Filed June 9, 1942 3 Sheets-Sheet 1 y 1949- T. D. GREGG 2,475,022

FLUID REACTION PROPULSIVE DEVICE Filed June 9. 1942 3 Sheds-Shoot 2 IN VENTOR Tues/min D. Grqqg ATTORN 1'. D. GREGG v 2 2,475,022 FLUID REACTION PROPULSIVB DEVICE 7 July 5, 1949.

3 Shuts-Sheet 3 Filed June 9. 1942 INVENTORY Patented July 5, 1949 I UNITED STATES PATENT OFFICE FLUID REACTION PROPULSIVE nnvron Tresliam D. Gregg, New York, N. Y.

Application June 9, 1942. Serial No. 446,355

20 Claims. 1

This invention relates to devices for producing motion by means of the propulsive reaction of jets or streams of fluid, and particularly to means for guiding the fluid so as to reduce the losses that characterize known devices of the let reaction type.

As is well known, the principal means of producing jets or streams of fluid are nozzles and screw propellers. Because of the viscous nature of all known fluids, the reaction of such jets is never as great as it would be if the fluid were free from viscosity. This is due to the fact that immediately surrounding and in contact with all such jets is a cylindrical body of fluid in the form of vortex rings which move down stream with a speed less than that of the .jet. Because of the nature of vortex motion those rings absorb a large part of the available energy of the jet. I

In the case of the screw propeller there exists, also, a system of helical vortex lines flowing from the tips of the propeller blades which exert a force upon the blades similar to that exerted by in which Fig. 1 is a side view of an airplane whose propeller is surrounded by an annular structure embodying the invention;v Fig. 2 is a fragmentary side-view of the said annular structure of Fig. 1; Figs. 2a, 2b, 2c, 2d, 2e and 2f are sectional views of the arrangement shown in Fig, 2; Fig. 3a is a front-view of a portion of the annular structure in which the annulus is shown in its shortest or retracted position and the other, designated Fig. 3b, representing it in its extended position; Fig. 30 represents in enlarged scale the devices shown in the cut-away portion of Fig. 3a to spread apart the sections of the annulus and thus increase its diameter; Figs. 4, 5 and 6 show the application of the invention to an automotive marine torpedo; Figs. '7 and 8 show its application to ship propulsion and Figs. 9 and 10 show its application to jet propulsion devices.

In Fig. 1, which shows the application of the invention to an airplane driven by two or more propellers, an annulus or annular structure, whose axis coincides with the axis of the propeller, partly or circumferentially surrounds the propeller. A radial cross-section through the wall of the annulus has the shape of an aerofoil or airplane wingpreferably of the Joukowski type. The inward e ge of the cross-section of the annulus would h ve the curvature of the upper or low-pressure surface of the aeroioil. The rounded or leading edge of the annulus section would be up-stream from the position of the propeller and the sharp trailing edge would be downstream therefrom. This is clearly shown in Fig. 2 which is a, fragment of the annulus, viewed from the side. In that figure, the inner edge, 4, of a radial cross-section is similar in shape to the upper or low-pressure surface of an aerofoil. The

rounded edge, 5, is up-stream from the propeller,

6, and the sharp trailing edge, I, is down-stream from the propeller as is also shown in fragmentary form in Fig. 2e.

In order to provide the necessary adjustability of the annulus, the reasons for which will later be fully stated, the annulus is composed of three sections, namely: a solid circular ring marked m in Figs. 3a and 3b containing the aerodynamic center c, indicated on Fig. 2, which term is clearly defined later on, the said ring extending from a short distance ahead to a short distance aft of that center and made rigid by struts l2 resting on the engine or the propeller shaft, as shown in Fig. 2, and by two or more tension members, such as l3 of Fig. 1, anchored to the wing or to the body of the plane; a down-stream part, made up of segments such as 8 and 9, supported by, and rigidly attached to, the solid ring and an up-stream part supported by the said solid ring, indicated by M in Figs. 2, 2b and 2d. The upstream and down-stream sections are each composed of a plurality of segments, those in the down stream section being indicated as mentioned above by 8 and 8 of Fig. 2. Each segment consists of two strips or plates preferably of hard sheet metal, one forming the inner surface and the other forming the outer surface of the segment. The strips of adjacent segments are so arranged that the strip of one segment overlaps the corresponding strip of its adjacent segment, the overlap of the inside strips being in the direction of the motion of the propeller the rigid ring section and, when in the retracted position, overlap also portions of the plates of the corresponding segments of the down-stream section of the annulus as indicated by the solid and broken lines 36 and 31 in Fig. 2 and I! in Fig. 2b, being so formed and shaped as to fit closely and press tightly the surfaces with which they coact. The adjacent segments of the forward part telescope laterall as indicated by the solid and dotted lines l8 and 19 of Figs. 2 and 2d.

From the foregoing it will be seen that this annulus, or annular structure, is in general like a frustum of a cone which is adapted to be lengthened or shortened by the forward or backward movement of the movable portion l4, and which is adapted to have its diameters varied by the lateral movement of the adjacent segments both of the forward and of the rearward sections of the annulus. The reason for providing such adjustability will now be stated.

In designing an annulus, in which this invention is embodied, there are two speeds to be considered, namely, the speed of the slipstream at the point of maximum speed with reference to the plane and the speed of the slipstream at that point with reference to the ground. The propulsive force or reaction is equal to the mass of fluid passing through the propeller disc in unit time multipied by the speed of the slipstream with reference to the ground. It can be shown from fundamental considerations that as the ratio of these two speeds changes, the diameter of the slipstream at its point of greatest speed also changes, so that the diameter of the annulus at its trailing edge, which must be equal to the diameter of the slipstream, must change correspondingly.- Moreover, to attain maximum effect with differing speed ratios both the length and shape of the aerofoil section must change. The required extent of those changes may be found by experiment or they may be calculated from the principles of hydrodynamics.

In the design of the annulus there are certain other factors to be considered. In all aerofoil sections a straight line drawn through the trailing edge of such section parallel to the line of flow producing zero lift and cutting the upper, that is, the low pressure surface is called the zero lift chord, and the point where it cuts the surface of the section is called the aerodynamic center of the section. The line d-d in Figs. 2

and 2b represents one position of this chord of zero lift. Itis essential that the conical locus of the zero lift chords shall have its large end up-stream and its small end forming the sharp trailing edge, down-stream from the propeller. It is essential also, for maximum effect, that the plane of the propeller disc s in Fig. 2 be as close as possible to the plane containing the aerodynamic center of the aerofoil section. The term "plane of the propeller disk will be understood if it is borne in mind that the lateral cross-sections of a propeller blade have the shape of an aerofoil having high pressure and low pressure sides, and that a zero lift chord may be drawn through each such cross section of a propeller. The point where that chord cuts the low-pressure side of the cross-section is the aerodynamic center of that section. The plane that passes through and contains the mean of the aerodynamic centers of all the cross sections of the blades of a propeller is the plane of the propeller disk. Otherwise stated, it is the plane of the propeller at which the pressure in the fluid rises suddenly as it passes through the propeller.

Again, it is essential, in order to eliminate helical vorticity, that the tips of the propeller blades be as close as possible to the surface of the annulus. Hence the diameter of the annulus in the plane of the aerodynamic center C, Fig, 2, and Fig. 2b is determined by the diameter of the screw. The diameter of the trailing edge is determined by the calculated diameter of the slipstream at its point of maximum speed.

The forward and backward movement of the front section H, and the enlargement of the diameter of the leading edge of this section and the diameter of the trailing edge of the rear or down-stream part, is effected by the operation of hydraulic rams which, as shown in the figure, are positioned within the shell or wall of the annular structure and are operated by the pressure of fluid conveyed to them by tubing from a central reservoir or reservoirs. The manner in which that is accomplished is as follows:

Fig. 3c shows the positioning of a ram for the "ment of two adjacent segments accompanying the adjustment of diameter of the rear part of the annulus. That ram consists of a cylinder 20 which is pivotally connected to a bracket 2| rigidly fastened to the metallic strip 22 of the segment 8 of the annulus. The piston 23 is pivotally connected to a bracket 24 rigidly connected to the inner metallic strip 25 of the adjacent segment 9, and the inner strips of which overlap the strips of segment 8. The fluid for the operation of the ram is fed into the orifice 26 in the pin holding the said cylinder to the bracket, by the tubing i shown in Fig. 2, and Fig. 2 As there indicated by the line a:--:c, the section shown in Fig, 3c is a cross-sectional view of the rear segments along the line :c:r. With an increase of fluid pressure the piston 23 will be moved to the right (as view in Fig. 3c) and, in consequence, the strips will bend outwardly and the overlapping of adjacent strips will be reduced and the diameter of the annulus will be increased. In order that the required bending may take place the strips or plates forming the segments of the rear part of the annulus must be curved in one direction only, namely, the radial direction, so that a cross-section normal to the axis of the annulus appears polygonal instead of circular. This is shown for example in Fig. 3b. Moreover, the upper and lower strips of each segment are not rigidly fastened together at their rearward ends but are designed to press firmly against each other, as is indicated by I in Fig. 2e. It should be noted that, as this flgure indicates, the trailing edges of these strips are ground thin. Now, as the movement of the rains causes the rear end of the annulus to increase in diameter, the ends of the upper strips, such as 22 and 25, move from the position D to the position D1 of Fig. 2 and Fig. 2e. Since the upper and lower strips of each segment are not attached to each other they will adjust themselves to the change in position of the upper strips corresponding to the above described change in diameter of the trailing edge of the annulus.

The position of the rams in the shell of the annulus is shown in Fig. 3a in which the rear portion is represented as being in part, cut away. The feed pipe individual to each ram would be connected to a main feed pipe running around the annulus as shown by I in Fig, 2f a d xt ding to the central reservoir.

The devices for the forward and backward movement of the forward part of the annulus.

Such a ram is shown in Fig. 20.

and for the lateral movement of the component segments required to effect the necessary changes in the diameter of the leadingedge of the annulus, and the manner in which those devices function will now be described. As previously mentioned, the plates constituting the walls of each segment of the forward portion press tightly against the upper and lower surfaces of the rigid supporting ring, as shown clearly in Fig. 2b. Within the said walls of'each segment is placed a ram the cylinder of which, indicated by 26 of Fig. 2b, lies in a slot n provided for it in the ring and is pivotally fastened to a bracket 21 by means of a tubular pin, the said bracket being firmly attached to the supporting ring of the annulus. The piston-he d, 28, of the ram is pivotally connected by a tu ular rod 34 to a bracket 29 in the lip of the forward part of the segment as shown in Figs. 2b and 2d. The said bracket may be attached to both the inner and outer plates of the shell and thus help to maintain its form. The cylinder at its forward end fits tightly around the tubular piston rod. The function of these longitudinal rams is to push forward the lip of the annulus to the desired length or to draw it back.

As the segments of the movable front section are pushed forward by the operation of the longitudinal rams, it is essential. at the same time to expand the segments laterally in order to increase the diameter of the annulus at its forward end. Such expansion is effected by rams operating laterally in the same general manner in The overlaps of the plates forming the interior surface of the segments of both'up-stream and 1. At the take-off an airplane provided with I this device can attain flight speed with a shorter run, or with a greater load than, with the same power, without this device.

2. After flight begins a plane provided with this device can accelerate to maximum speed at a faster rate than it can. with equal power, withwhich the similar rams move the segments of the rear part of the annulus, as previously described. Its cylinder 30 is pivotally connected by a partly tubular pin 38 to a bracket 3| fastened to the shell of the forward part of the segment. The piston 32, shown in fragmentary form, is connected to a bracket (not shown) in the adjacent segment. The fluid for actuating the lateral ram in Fig. 2c enters the cylinder through an orifice in the forward end of the pin connecting the cylinder to the bracket from the tube 33. As shown in Fig. 2a, which is a conical section taken along the line y- 'y of Fig. 2, the tube 33 is connected to the bearing pin of the bracket 29, which bearing pin is bored so as to continue the fluid connection to the tubular rod 34 which is connected to the piston 28. The fluid for the forward actuation of the longitudinal ram is supplied from a reservoir connected to tube 3 of Fig. 2a, passing thence into the tubular pin 3' supporting the cylinder in the bracket 21, thence into the clearance chamber 46 of the cylinder where it presses upon the rear face of the piston a part of the fluid thence passing into the tubular rod 34 to actuate the lateral ram in Fig. 20, as above described. The fluid for the backward motion of the longitudinal rams is supplied from a central reservoir to tube 2 shown in Fig. 2a, thence to tube 35 which passes through a hole P in the supporting ring to the forward face of the said ring and thence to a hole in the wall of the cylinder where it exerts pressure upon the forward face of the piston 28 causing it to movebackward when the pressure upon the rear face is relieved.

out this device. 4

3. A plane having this device can climb at a steeper angle with the same load or at the same angle with a greater load, or at greater speed, with the same power than without this device.

4. With this device, the greater the total thrust in relation to the speed of flight the greater the ratio of total thrust to thrust of the propeller alone and the greater the total thrust per brake horsepower. Thus such a plane has a greater drag capacity per horsepower which could be utilized for towing other planes or gliders.

5. The propeller works at greater efliciency with this device than without it.

A propeller of small diameter with this device would have-an overall efficiency equal to that of a larger propeller without the device and also would have a lower tip-speed and greater permissible width of blade.

The mode of operating the mechanism employed in this invention in order to accomplish the desired result has been so fully described hereinbefore' ,that further detailed description seems unnecessary. The application of fluid pressure to the plurality of rams may be controlled manually or may be automatically regulated by flight conditions.

The aforedescribed apparatus is essential for best operation under variable speed conditions. Where the speed is constant, the adjustability feature may be dispensed with. The application of this invention under the latter condition is shown in Figs. 5 to 10 inclusive.

Figs. 4, 5 and 6 show the use of the invention in connection with an automotive marine torpedo having two oppositely rotating propellers, which run at substantially constant speed. Fig. 4 is I a general view of the torpedo showing the device The lateral rams are connected by their.

at a. Fig. 5 is an enlarged view of the stern showing an axial cross-section of the annulus b supported by the rudder vanes c and four streamlined vanes d. The line .E--E marks the plane of the aerodynamic center and the lines FF are zero lift chords. Fig. 6 is a transverse section on the line L-L.

Fig. 7 shows the application of the invention to propel a boat as, for example, by outboard motor not shown, of which Fig. 8 is an axial View looking in the direction M-M. Figs. 9 and 10 show the use of the invention in connection with a nozzle propelled jet as a propulsion device. Fig. 9 is a cross-section along the axis of the device showing the, annulus G, the nozzle H and four supporting struts J which hold the annulus rigidly in position with respect to the nozzle. The

7 line E-E marks the aerodynamic center and F-F, the zero lift chords. Fig. 10 is an up-stream view of the device. In the application of this invention to a nozzle propelled jet, there are two points not hereinbefore mentioned that should be observed:

1. The discharge or exit plane of the nozzle which is its plane of actuation, must coincide as nearly as practicable with the plane of the aerodynamic center of the aerofoil ring section as located by the position of the zero lift chords, indicated by the lines FF in Fig. 9.

2. Suflicient space must be provided within the area between the nozzle and the inside surface of the annulus for the entrance of the entrained fluid; that determines the diameter of the annulus at the aerodynamic center.

A simple illustration of the reason for placing the plane of actuation, that is, the point of application of the driving force, at the aerodynamic center of the aerofoil section is as follows:

The aerodynamic center of a sphere, such as a baseball, in motion is the point on its forward surface intersected by the line of motion of its center. This line contains the zero-lift chord" of the ball. If the batter can hit the ball at this point with the center of percussion of his bat moving along the same line of motion as the ball he will produce the greatest possible relative speed of rebound. Should he hit the ball below or to the right or left of this point he knocks a "foul or pop fly: while if he hits the ball above this point he knocks a grounder."

To carry the analogy further; the center of percussion of the bat corresponds exactly to the plane of aerodynamic centers of the propeller blades, i. e., the "propeller disk, or the plane of actuation of the nozzle.

It is understood that, while reference has been made to the use of metallic strips or plates in the formation of the segments of the annulus, the invention is not so limited since strips of plastic, rubber, wood or other material having the required stifiness and resiliency may lac-employed. I

without departing from the spirit and scope of the appended claims.

What is claimed is:

1. In a propulsive system, the combination with a screw propeller having blades whose transverse cross-sectional form is that of an aerofoil and whose longitudinal axes are substantially perpendicular to the propeller axis, of an annular structure coaxial with the said propeller, the wall of the said structure having the longitudinal cross-sectional shape of an aerofoil, the low pressure side of the aerofoil section of the said structure being inward, the said propeller being so positioned that the plane containing the mean of the aerodynamic centers of all the propeller blade sections, constituting the plane of the propeller disc, shall substantially coincide with the plane containing the aerodynamic centers of the longitudinal aerofoil sections of said annular structure.

2. In a propulsive system, the combination with a screw propeller having blades whose transverse cross-sectional form is that of an aerofoil and whose longitudinal axes are substantially perdendicular to the propeller axis, of an annular structure coaxial with the said propeller, the longitudinal section of the wall of the said structure being in shape similar to an aerofoil, the low pressure side of the aerofoil section of the said structure being inward toward the longitudinal axis of the annulus, the said structure being so formed that the conical locus of the zero lift chords of the cross-sections shall have its large end up-stream from the position of the said propeller and its small end down-stream therefrom and passing through and forming the trailing edge of the said annular structure and the said propeller being so positioned that the plane containing the mean of the aerodynamic centers of the propeller blade sections, constituting the plane of the propeller disc, shall substantially coincide with the plane containing the aerodynamic centers of the aerofoil sections of the said annular structure.

3. In a propulsive system, the combination with a propeller having blades whose transverse crosssectional form is that of an aerofoil and whose longitudinal axes are substantially perpendicular to the propeller axis, of an annular-structure surrounding the said propeller and coaxial therewith, the wall of the said structure having the cross-sectional form of an aerofoil with the low pressure side toward the center of the annulus, and the separation between the inner surface of the structure and the tips of the propeller blades being just sufllcient to provide safe clearance under operating conditions the said propeller being so positioned that the plane containing the mean of the aerodynamic centers of the propeller blade sections, constituting the plane of the propeller disc, shall substantially coincide with the plane containing the aerodynamic centers of the aerofoil sections of the said annular structure.

4. An annular structure for encircling an airplane propeller having blades whose transverse cross-sectional form is that of an aerofoil and whose longitudinal axes are substantially perpendicular to the propeller axis, the said structure comprising a rigid annular midsection adapted to be supported by the said airplane, and substantially coinciding in position to the plane containing the mean of the aerodynamic centers of the propeller blade sections, constituting the plane of the propeller disc, an up-stream section comprising a plurality of segments slidably connected to the annular midsection adapted to be moved forward and backward longitudinally and a down-stream section also comprising a plurality of segments connected to the said annular midsection, the extremities of which latter segments are adapted to be bent radially whereby the diameter of the rear orifice of the said annular structure may be increased or decreased, the axial cross-section of the said structure being that of an aerofoil with the low pressure side facing inwardly toward the longitudinal axis.

5. An annular structure for encircling a propeller having blades whose transverse cross-sectional form is that of an aerofoil and whose longitudinal axes are substantially perpendicular to the propeller axis, the said structure comprising a rigid annular midsection, the middle plane of which coincides substantially with the plane containing the mean of the aerodynamic centers of the propeller blade sections, constituting the plane of the propeller disc, an up-stream section comprising a plurality of segments movably connected to the midsection, means to move the segments of the up-stream section forward and backward, a down-stream section also comprising a plurality of segments connected to the said midsection and means to move the extremities of the latter segments laterally with respect to each centers of the propeller blade sections, constituting the plane of the propeller disc, a segmental forward section movably attached to the said rigid midsection, the said forward section constituting an annular up-stream orifice of said structure, means to alter the diameter of the said up-stream orifice, and a segmental rear section also attached to the said rigid midsection, the said rear section constituting an annular down-stream orifice of the said structure, and means adapted to bend the segments of the said rear section whereby the diameter of the said orifice may be increased or diminished.

'7. An annular structure for encircling a propeller comprising a rigid midsection and a segmental forward section movably connected to said midsection, and a group of double-acting hydraulic rams, each comprising a piston and a cylinder, each ram being connected to one segment of said forward section and to the said midsecw tion, the said ram having means by which fluid pressure may be applied to the said piston to efiect the forward and backward movement of the said forward section whereby the length of the said annular structure may be increased and decreased.

8. The structure defined by claim '7 further characterized by a second group of hydraulic rams each individual to and connected to an adjacent pair of overlapping segments, the said rams having means to apply fluid pressure thereto whereby the degree of overlap may be increased or decreased and the diameter of the said forward section decreased or increased.

9. In a propulsive system, the combination with means to project a jet of fluid, of an annulus encircling the said jet, the wall of the said annulus having the cross-sectional shape of an aerofoil with the low pressure side of the said aerofoil positioned inward toward the longitudinal axis of the said annulus the plane of actuation of the said jet coinciding with the plane of the aerodynamic centers of the aerofoil sections.

10. The structure defined by claim 9 further characterized in that the cross-section of the Wall of the annulus has the shape of an aerofoil with a high lift to drag ratio.

11. In a propulsive system the combination with means to project a jet of fluid of an annulus encircling the said jet and coaxial there with, the wall of the said annulus having the longitudinal cross-sectional shape of an aerofoil with the low pressure side positioned inwardly toward the said jet, the plane of actuation of said jet projecting means substantially coinciding with the plane of the aerodynamic centers of the sections of said aerofoil, and the space between the said jet projecting means and the inner surface of said annulus being sufficient to provide for the inflow of the induced fluid.

12. In a propulsive system the combination with means to project a jet of fluid of a conical annulus encircling the said jet and coaxial therewith, the wall of the said annulus having the longitudinal cross-sectional shape of an aerofoil with the low pressure side positioned inwardly toward the said jet, the plane .of actuation of the jet projecting means substantially coinciding with the plane of the aerodynamic centers of said aerofoil, the locus of the zero lift chords of the said aerofoil sections of the said annulus forming a cone with its large end up-stream from the said jet projecting means, and means to alter at will the diameters of the up-stream and the downstream ends of the conical annulus.

13, An annular structure for encircling a propeller comprising a rigid midsection and a segmental forward section connected to said midsection, a group of hydraulic'rams, each compris ing a piston and a cylinder, each connected to a segment of the forward section and to the midsection, a second group of hydraulic rams each individual to and connected with an adjacent pair of overlapping segments, all of the said rams having means to apply fluid pressure thereto whereby the longitudinal movement of the segments of the forward section and the lateral movement of the overlapping segments may be efiected to vary the length and the cross-sectional diameters of the annular structure, the said structure being further characterized in that the fluid applied to the rams of the second group is conveyed thereto through the tubular piston rods of the rams of the first group, or by independent means.

14. In a marine propulsive system, the combination with a propeller having blades whose transverse cross-sectional form is that of an aerofoil and whose longitudinal axes are substantially perpendicular to the propeller axis, of an annular structure encircling the said propeller, the wall of the said structure having the longitudinal cross=sectional shape of an aerofoil with its low', pressure side positioned inwardly toward the longitudinal axis of the annular structure, the said propeller being so positioned that the plane containing the mean of the aerodynamic centers of the propeller blade sections, constituting the plane of the propeller disc, shall substantially coincide with the plane containing the aerodynamic centers of the-said aerofoil sections of the annular structure, and the locus of the zero lift chords of the said aerofoil sections shall form a cone having its large end up-stream from the propeller.

15. In a propulsive system, the combination with means to project a jet oi. fluid of an annulus encircling the said jet and coaxial therewith, the wall of the said annulus having the longitudinal cross-sectional shape of an aerofoil having a high lift to drag ratio and a keen trailing edge with the low pressure side positioned inwardly toward the jet, the said jet projecting means being so positioned that its plane of actuation substantially coincides with the plane of the aerodynamic centers of the aerofoil sections, and the surfaces of the said annulus being composed of a plurality of longitudinal segments, the edges of which overlap, and means connected to the said segments to vary at will the degree of overlap of adjacent segments whereby the diameters of the said annulus perpendicular to its of an aerofoil with its low-pressure side inward toward the axis of the annulus, the said structure being so formed that the conical locus of the zerolift chords of the aerofoil sections shall have its large end up-stream from the propeller and its small end down-stream, passing through and forming the trailing edge of the annular structure, the surfaces of the annulus being composed of a plurality of longitudinal segments, and means to adjust the relativeipositioning o! the adjacent segments whereby the diameters of the annulus, perpendicular to its longitudinal axis, and the length and curvature of its segments may be altered.

17. An annular structure for encircling a propulsive jet of'fluid at the plane or actuation thereof, the wall of the said structure having the cross-sectional form of an aerofoil having high lift to drag ratio and having a keen trailing edge, the low pressure side of which is positioned inwardly toward the longitudinal axis of the annulus, the said structure being segmentally adjustable throughout the maior part of the circumference.

18. In a propulsion system, the combination with a series of propellers, each having a plurality of blades, each having the transverse crosssectional shape 01' an aerofoil and its longitudinal axis substantially perpendicular to the propeller axis of an annular structure co-axial with the said propellers, the wall oi. the said structure having the cross-sectional shape of an aerofoil, the low pressure side being inward, the said propellers of the series being so positioned that the plane containing the center of gravity of the several propeller disks of the series shall substantially coincide with the plane containing the aerodynamic centers of the aerofoil sections of the annular structure.

19. An annular structure for the encirclement of an airplane propeller, the said structure simu lating the form of a hollow frustrum of a cone having its larger diameter up-stream, and its smaller diameter down-stream from the position of the propeller, each longitudinal crosssection or the wall of the said frustrum having the form of an aeroioil with its low-pressure side inward toward the axis of the propeller, the surfaces of the wall of the said trustrum being composed of a plurality of longitudinal segments, the edges of each segment of each surface overlapping the adjacent segments of that surface, and means connected to the said segments to vary at will the degree of the said overlap whereby the diameter of the various cross-sections of the said frustrum and the length and curvature of the said segments may be altered.

20. In a thrust producing means the combination of a rotatable airscrew, a duct surrounding the airscrew and having a plurality of hinged overlapping sections defining a portion of said duct which is cylindrical in cross-section, and means for changing the efiective diameter of said portion of said duct, while maintaining the same approximately cylindrical in cross-section, comprising mechanism for simultaneously rocking each of said sections on its hinge while maintaining the overlapping portions thereof in overlapping relationship.

TRESHAM D. GREGG.

REFERENCES CITED The following referenices are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 442,614 Dock Dec. 16, 1890 1,327,543 Funk Jan. 6, 1920 1,375,601 Marize Apr. 19, 1921 1,493,157 Melot May 6, 1924 1,954,437 Washburne Apr. 10, 1934 2,030,375 Kort Feb. 11, 1936 2,123,657 Munk July 12, 1938 2,173,330 Gregg Sept. 19, 1939 FOREIGN PATENTS Number Country Date 316,816 Italy Apr. 16, 1934 797,202 France Apr. 23, 1936 

